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I know that when you install hardwood flooring, you need to leave a small expansion gap along the walls. I understand that this is to allow the wood some space to move when the wood expands due to humidity and temperature fluctuations.

My question is why the expansion gap works, especially with floors that are nailed or stapled to the subfloor. If each board is nailed down, how is an expansion gap along the edges of the room going to help when the wood expands? Wouldn't each board just collide into the ones alongside it?

Plus, even if you have a floating floor where most of the boards are not nailed down, the ones along the edge of the room typically would be nailed...so wouldn't they be immobile and prevent any movement into the expansion gaps?

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It's more accurate to say that the wood fibers swell rather than the wood moves. A board which is nailed down will still swell and contract around the nails, generally leading to a larger hole and squeaky floor. The nails hold the floor down and prevent gross movements, such as the floorboards sliding around underfoot.

Another key point to grasp is that boards swell along the growth rings in an amount proportional to their size. For practical purposes that means they really only have noticeable changes in width and it's a couple of percent of the width of the board. Adding an extra 1/64" to any individual board isn't a big deal, but it can add up across a lot of boards.

The gap is really about averting disaster. You're a board soaking up humidity. The tongue and groove joint gets a bit tighter, taking up some of the slack. Nail holes enlarge or nails bend slightly as you shove your neighbor aside an imperceptible amount. The floor remains flat and all is well, unless you're tight against the wall to begin with. In that case boards swell, and they press on their neighbors. They can't move sideways or down, so they go up. Boards cup, edges rise, the floor is warped.

  • Exactly. And they’re usually nailed into “sleepers”. – Lee Sam Apr 12 '18 at 2:34
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Since I cannot comment right now (not enough reputation), I will write my understanding and clarify the OP question here.

As pointed out by Matthew Gauthier, even if the hardwood floor is nailed down, it still swell and contract according to its environment. Let's suppose the subfloor on which the hardwood floor is nailed down has very negligibly small expansion coefficient (this is the initial simple model, more complications can be considered later). During the expansion or contraction of hardwood floor, since the subfloor is not changing, there must exist at least one point on the hardwood floor that is not moving relative to the subfloor. Let's assume this stationary point to be in the center of the room (the conclusion will not be affected if this point is near the room wall). The further away from this point, the more expansion or contraction movement at the other points. So here is the conclusion: the gap near the wall will indeed help to avert the disaster, but the hardwood floor needs to overcome the holding force from all those many nails first before the possible disaster near the room wall. This would also lead to larger nail hole when the nail is further away from the stationary point. The hole could be near the size of the expansion gap which is of ~ 1/4 inch.

Please point out any flaws in the above logic.

  • I don’t understand your correlation between the distance from “the hardwood floor needs to overcome the holding force from all those many nails...” to “This would also lead to larger nail hole when the nail is further from the stationary point.” Hmmm...where did “larger nail hole...” come from? Larger nails? – Lee Sam Jul 31 '18 at 16:54
  • At the day of installation, lets assume all nails are straight (the conclusion won't be affected if the nails are tilted) and penetrate through both hardwood floor and subfloor, leaving two holes on the hardwood and on the subfloor. All these holes are exactly on top of each other. Days or months later, due to expansion or contraction of hardwood floor, those holes won't align with each other any more, and the misalignment is larger when the hole is further away from the stationary point. The larger misalignment can lead to either larger holes on hardwood or subfloor, or deformed nails. – Jasper Jul 31 '18 at 17:05
  • Ahhh...I see. Actually, wood flooring nails are installed in the “tongue” of each board at about a 45 degree angle into a loose lying “sleeper”. The sleeper is a piece of wood lying on the subfloor. So, when each wood board expands, (because the wood fiber expands) the nail does not “bend” or deform the subfloor...it merely “moves” the sleeper, slightly. The head of the nail stays EXACTLY in the same position relative to the sleeper and the board it is nailed into. The expansion problem occurs due to the wood fibers growing. If you multiply that growth in each board times the number of – Lee Sam Jul 31 '18 at 17:21
  • Boards then you’ll often get a total expansion of 1”-2”. If you don’t leave a gap at the wall, the entire floor will push against the wall until the floor bows up...lifting the sleepers too. I’ve see a floor bowed up 24” in the center of a room (gymnasium). – Lee Sam Jul 31 '18 at 17:26
  • Could you please explain more about what is exactly sleeper? Let's say the room subfloor is consisted of plywood screwed onto joists. Do you mean that you need to put a sleeper first on the plywood, and then nail down the hardwood floor on the sleeper? If so, what is wood type of the sleeper? How thick is the sleeper? How do you attach sleeper onto plywood? – Jasper Jul 31 '18 at 17:54

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